A rear suspension for a two wheel vehicle is utilized to isolate the operator, the rider, from the impacts and undulations of the terrain whilst maintaining rider control. There are various phenomena associated with the suspension of a two wheeled vehicle such as a bicycle; these tend to be a result of rider input or the movement of the rear wheel through its suspension travel. These are discussed briefly below.
Various proposals for bicycle rear suspension systems have been made, including various designs of four bar linkage type mechanism which typically comprises the frame (chassis), a rear triangle, and upper and lower links. An example of such a proposal is given in WO 2005/030564. Four bar linkages allow the rear wheel to move relative to the frame in a manner that will isolate the frame and rider from bumps and impacts. Shock absorption is usually provided by a spring, often with a damper to control the spring's oscillation. The spring is anchored to the frame and one of the linkages or rear triangle and acts upon it when the rear wheel moves.
A problem that the Applicant has recognized with four bar linkages is that of instant centre migration. The term instant centre, also known as the virtual pivot point, refers to the location of the centre point of the curvature of the wheel path at any point throughout the suspension movement. This point is found on a four bar linkage by extrapolating the straight line between the pivots on the upper and lower links. Where these two extrapolated lines cross is the instant centre. It can be seen that as the rear suspension moves and the position of the upper and lower links changes the position of the instant centre moves. The instant centre can migrate throughout the wheel travel and can cause unwanted handling phenomena such as an increase in chain length of the tensioned section of the chain, this leads to an effect known as pedal kickback where the alteration in distance between the front and rear sprockets forces one of the sprockets to rotate. On a bicycle the front sprocket tends to rotate causing the pedals to turn either backwards against the direction of power input, or to rapidly turn forwards due to a shortening of the chain length. This unbalances the rider. In a similar manner, pedal induced forces and braking forces can cause squat and jacking respectively. This is where the tension through the chain causes the wheel to rise relative to the chassis, this in turn leads to the effect of pedal-induced bob, and this means that rider energy is wasted as vertical movement rather than into forward motion. Braking forces induced from the contact patch can transmit forces through the suspension causing the wheel to be forced down relative to the frame. This is known as jacking.
On most bicycles, the rear wheel contact patch tends to move forwards as it rises throughout its suspension travel. The forward movement is usually in an almost opposing direction to the force being transmitted from a bump. When the contact patch hits a sharp rise in terrain, a force is transmitted through the wheel that will try to move the wheel upwards and slightly rearwards. If the wheel is restricted to travelling forwards causing a reduction in wheelbase as it rises, then it equates to a loss of energy over rough ground which may unsettle a rider by trying to pitch them forward. An alteration in wheelbase also changes the handling characteristics. A reduction in wheelbase usually leads to a less stable handling vehicle.
In the suspension system disclosed in the aforementioned WO 2005/030564, the design is intended to give an anti-squat curve in the initial part of the suspension travel, which decreases as the suspension moves deeper into its travel. The Applicant has determined that this arrangement gives uncontrolled movement of the rear axle path and indeed it is noted that at the far end of this travel the rear axle ends up forward of its initial position. This is undesirable, inter alia as it alters the chain length. The Applicant has also determined that the arrangement disclosed therein gives a large degree of instant centre migration, which is exacerbated by being relatively flat and horizontal. This is disadvantageous as set out above and contributes to a strong coupling between pedal forces and suspension movement.